Irrigation system controllers deliver water for set periods of time on a fixed schedule. The frequency of delivery can be manually adjusted and the time period can be adjusted. For example, the frequency can be set at once every three days and the time period can be set at one hour. This schedule will remain in effect to supply an average need until changed by the user. Also, it is possible to adjust the water flow rate during a water delivery period, i.e., the amount of water being delivered per minute.
If there is a period of cool overcast days with some local showers it may be that the average three day allowable soil moisture depletion will not occur for seven rather than three days following the previous irrigation cycle. Regular fixed irrigation schedules will then overwater, causing delayed plant growth, a boost in fungus growth and a waste of water.
The user can make a guess as to the degree of moisture depletion and adjust manually by changing the frequency or duration of the irrigation cycle, or the rate of delivery of water during an irrigation cycle, but this requires constant monitoring of micro-climate changes and seasonal changes. The soil moisture depletion is measured by and coincides with cumulative evapotranspiration numbers--a measure of the inches of precipitation needed for replacement of the depletion, occurring over a given period of time, usually of a turf grass field as a standard. The desired situation is a matching of the amount of water delivered, whether delivered naturally or by an irrigation system, with the evapotranspiration of a particular field planted with a particular crop (The term crop as used herein includes not only fruit and vegetable products which might be intended for human or animal consumption but also decorative crops such as lawns, shrubs, trees and the like). The prior art has not provided an adequate and inexpensive answer to this problem.
U.S. Pat. No. 4,770,345 to Ross, Jr., et al discloses a simple mechanical system which utilizes a bucket to receive a small portion of the irrigation water flow and to act as a counterweight which shuts off an irrigation valve when sufficient water has been collected in the bucket. The system of this patent will serve to prevent activation of an irrigation system when there is sufficient rain or sufficient into-the-air-sprinkled irrigation water to fill the bucket. The system is exposed and can be readily damaged, for example by vandals. There is no provision for adjusting the rate of evaporation of the captured water and no control of the amount of water which can enter the bucket. It is not adaptable for irrigation systems which do not spray irrigation water into the air but instead flow it through ditches or deliver it below the surface of the soil.
U.S. Pat. No. 4,190,201 of Geiger discloses a misting control that includes a liquid collecting and sensing element that has a screen element which collects only a thin film of water. The sensing element pivots about an axle in response to the presence of the thin film of water and activates or deactivates the misting system via a terminal block.
U.S. Pat. No. 2,965,117 to Gallacher discloses an irrigation control system having a receptacle that is mounted for vertical movement in response to the level of water in the receptacle. Movement of the receptacle is also dependent upon a spring mounted beneath the receptacle. The receptacle is filled when the irrigation system is on via water from a pipe. The receptacle may also be able to collect rain water.
Other arts have utilized pivotally mounted tanks or containers balanced at one end by a spring to determine the level in a tank. U.S. Pat. No. 4,883,200 to Miller, et al and U.S. Pat. No. 4,063,605 to Graham each disclose such systems. Such systems are not used, however, for control of activation of irrigation systems. The Miller, et al patent is concerned with a level indicator for a thermoplastic melting apparatus and the Graham patent is concerned with a level indicator for a fluid power transmission system.
There are two factors which make up evapotranspiration (ET). These factors are evaporation (E) from the soil and from plant surfaces and transpiration (T), i.e., the evaporation that takes place within plant leaves and the vapor that diffuses into the air through the stomata on the leaf surfaces. The ET on any particular day is a function of the weather, season, crop age, crop size and surface roughness. Generally, values for reference evapotranspiration (Eto) are available for different locations throughout the country. The ETo approximates the actual ET of a large field that is not water stressed. ETo can be directly measured but is often calculated from weather data. Other information which is available relates to a crop coefficient (Kc) which will allow values to be determined for the daily evapotranspiration for a particular crop (ETc). The relationship is ETo x Kc is equal to ETc. Publications which discuss the determination of daily reference ETo include Determining Daily Reference Evapotranspiration (ETo), Leaflet 21426, Turfgrass Evapotranspiration Map Central Coast of California, Leaflet 21491, and Estimating Water Requirements Of Landscape Plantings--The Landscape Coefficient Method, Leaflet 21493, all published by the Cooperative Extension University of California, Division of Agriculture and Natural Resources, the contents of each of which is incorporated herein by reference.
It will be noted that the prior art systems for controlling irrigation have generally not been adjustable or designed to take into account the evapotranspiration of a specific region having a specific crop growing thereat.